Schistosoma mansoni, a human blood fluke parasite, causes a disease of major health importance in many parts of the world. The principal cause of pathology of the disease is a host reaction against products secreted by the schistosome egg. In addition to the effect of secreted products in stimulating the host's immune response, the egg probably also produces proteolytic enzymes which may be involved in the penetration of these eggs through host tissues. We have isolated a thiol-dependent, acidic pH-optimum protease from schistosome eggs and have begun to purify and characterize this enzyme. The purpose of these studies is to purify this enzyme to homogeneity, and to characterize the enzyme with respect to substrate preference, action on connective tissue substrates, and immunological cross-reactivity with proteases from adult schistosomes. Studies on the presence and characteristics of proteolytic enzymes from the miracidia of S. mansoni will also be initiated.